difficult

adjective:needing great effort or skill to accomplish; not easy to do or understand
- The calculus problem was so difficult that the whole class had to work together to solve it.
adjective:requiring a lot of mental or physical exertion; demanding
- Climbing the steep mountain trail proved to be difficult for the inexperienced hikers.
adjective:hard to endure or tolerate; causing discomfort or inconvenience
- Living in extreme cold can be difficult for people who are not accustomed to the weather.
adjective:having a stubborn or uncooperative nature; not giving in easily
- She was a difficult child, often refusing to follow instructions.